Oh wow. I didn't realise DoF had been a topic in this thread since November.
Anyone have a URL handy for a good DoF description / tutorial? The one at
Wikipedia contains good info but is less-than-straightforward.
We've got several good descriptions / discussions starting about
here and in the next posts after that.
I like to think of the phrase "Depth of Field" as being short for something like "The depth of the 'proportional range' of acceptable sharpness." With any particular settings, there is one distance at which objects are in focus and there is a range of nearby distances that are 'acceptably sharp'. Keeping the same lens, focal length and aperture but changing the focal distance keeps the same proportion of ranges acceptable even though the distances to the objects changes, e.g. say I'm focused at 10' and my DoF is 9-12', refocusing at 20' is going to give me a DoF of about 18-24'.
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<an aside>
It *roughly* works like that. Different lenses and lens designs work differently. It's also not a linear scale, but is rather constant proportion of the lens' focal range. Using a photo from Wikipedia,
this lens is focused at about 1.5 m and set at f/11, so everything in the 1-2 m range is going to be acceptable. If we refocused to 4-ish m, then everything in the 1.5 m-∞ range is going to be acceptable. As you change the focal distance on the lens the range of measured distances to the objects that are 'acceptable' changes, but the proportion for the lens stays constant.
</aside>
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I'm not certain, but I think the following is correct.
That there is a range is mostly (or entirely) due to how our vision works. How deep or shallow the DoF can be is mainly dependent on how light bends in a particular lens.
Shorter lenses are bending the light a lot when the irises are wide open and results is greater variance on your imaging surface. Stop 'em down and 'the proportion of their focal range that produces acceptable sharpness' increases because it's using just the center of the lens and bending the light less and creating less variation on the imaging surface. In long lenses the same thing happens, but it is less apparent because they inherently have smaller depths of field to begin with.
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<another aside>
This is different from how two different lenses with the same basic design will have different depths of field at the same settings, say two 28 mm'ers at 2 m at f/4. In this case, the difference is in the optical design and quality.
</aside>
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... Clues, please! Do I need a full frame camera? Do I just need to shoot longer focal lengths with big (f/2.8) apertures??
The short answer is no. Don't spend your time / money / energy there. Spend it on seat time. Some additional tips are to focus in front of your subjects so that the subject is still in the DoF and to set up your shots with the backgrounds further away. These will both put more of your backgrounds out of the DoF and make them less focused.
Like, look at the Zoriah photos. For the single child, I think the focus was on the top of the head but because of the DoF the eyes are 'close enough' and the background is slightly more defocused than it would have been if it'd been focused, say, on the nose. In the three children photo, the design focus is on the tallest child, but the image focus is on close side of the middle child's head. This allows all three children to be 'in focus' but makes the background people a little more defocused.
On the Chinese New Year photos, the 70-200mm has as smaller proportional range than the 50 mm prime, so even by opening up more than the other person could have you probably still had a longer / deeper / larger DoF. They just behave differently. I would want to compare your images cropped down to the same field of view before dismissing the shots with f/1.8 though. To get images that really would have pleased you it might have been better to find a different position / location and to have shot different things because you were using different tools.
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<yet another aside>
When I'm out shooting -- that's working to create interesting images as opposed to 'taking pictures' -- it's a trip. I'll see a scene, static or dynamic, and then fly. I'll start moving around taking in as much of the scene as I can, looking for compositions, lighting, shooting positions, subject dynamics. Often I'll find that where I want to be to shoot something ... or where I want to wait to shoot something that's going to happen ... will be in an inaccessible spot like the middle of the street or floating thirty feet in the air. Sometimes I can't get the shot I want because I have the wrong camera or wrong lens with me. *Then* in any case it's my job to find the best I can for the tools I have with me and my physical limitations.
</aside>
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For the idea of using a full-frame camera to get more / better defocusing, it can make a difference, but you're looking at a < 1% change when you already have the real tools in your hands. I'd liken it to wanting to get faster track times by improving my aerodynamics instead of learning better body positioning and appropriate lines. Like a lot of things it's not the tools you have but how you use them.
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FWIW, here's my post about the bellows:
http://www.bayarearidersforum.com/forums/showpost.php?p=4569850&postcount=611
and the page with more info:
http://homepage.mac.com/mosquito/20081203-b-Bellows/index2.html
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